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The effectiveness of a 20 mph speed limit intervention on vehicle speeds in Bristol, UK: A non-randomised stepped wedge design

Bornioli, Anna; Bray, Isabelle; Pilkington, Paul; Bird, Emma L.


Anna Bornioli

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Issy Bray
Associate Professor in Public Health (Epidemiology)

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Emma Bird
Senior Lecturer in Public Health


© 2018 Elsevier Ltd Twenty mph (32.2 km/h) speed limits across urban areas are becoming a widespread tool for public health and road danger reduction globally. Determining the effectiveness of these interventions on motorised vehicle speeds is a crucial first step in any logic model that seeks to associate 20 mph speed limits to improved health and wellbeing outcomes. However, little is known about how the introduction of 20 mph limits affects speeds. This paper presents the findings from a novel comprehensive academic evaluation of the adjusted effects of a 20 mph sign-only city-wide intervention on vehicle speeds. This is based on a natural experiment that took place in Bristol, UK. Applying a quasi-stepped wedge design, speeds of 36,973,090 single vehicles, recorded by Automatic Traffic Counts before and after the 20 mph intervention, were analysed. Generalized linear mixed models were used to control for confounding variables. Results showed an unadjusted speed reduction of 4.7 mph (7.56 km/h) and an adjusted speed reduction of 2.66 mph (4.28 km/h) over two to three years. Some variability due to time variables was detected, with speed reductions being larger during night time, at weekends, and in summer months. The roads that did not receive the 20 mph intervention also saw a small reduction in speed (0.03 mph). The findings indicate that the sign-only 20 mph intervention was successful in lowering individual vehicle speeds. Policy makers are encouraged to implement a careful monitoring of the effects of 20 mph speed limit interventions on vehicle speeds in order to enable a meaningful evaluation of potential public health benefits.


Bornioli, A., Bray, I., Pilkington, P., & Bird, E. L. (2018). The effectiveness of a 20 mph speed limit intervention on vehicle speeds in Bristol, UK: A non-randomised stepped wedge design. Journal of Transport and Health, 11, 47-55.

Journal Article Type Article
Acceptance Date Sep 26, 2018
Online Publication Date Oct 9, 2018
Publication Date Dec 1, 2018
Deposit Date Sep 28, 2018
Publicly Available Date Apr 10, 2020
Journal Journal of Transport and Health
Print ISSN 2214-1405
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 11
Pages 47-55
Keywords 20 mph speed limit, road danger reduction, traffic speed, stepped wedge trial, United Kingdom, public health evaluation
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Additional Information Additional Information : This is the author's accepted manuscript. The final published version is available here:


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